Crosstalk interference between the receiving and the transmitting paths of different transceivers whose communications media are in physical proximity to one another, such as is often experienced between different wires in a telephone wire bundle, is well known. The crosstalk in a telephone wire bundle depends on the number of interfering lines, and increases as the bandwidth that the signals occupy increases. In a modem pool environment where streams of data are distributed to many lines within a single bundle, and where the bundle is used exclusively by the modem pool, the crosstalk that the receivers need to overcome is mainly generated by the transmissions that the modem pool itself generates. Since such a system has access to its own transmission characteristics, such information may be used to cancel the interference that leaks into the receivers, thus decreasing the noise floor of each receiver.
In classic crosstalk cancellation, a transmitter transmitting via one wire or wire grouping (e.g., twisted pair) affects the receiver receiving via another wire or wire grouping. A hybrid circuit separates the received signal from the transmitted interfering signal, but since the hybrid cannot completely separate the transmit path from the receive path, some of the transmitted signal leaks into the receiver and becomes an interfering signal. A canceller then filters out the effect of the interfering signal, resulting in a “cleaned” received signal. For a single modem, this problem may be addressed using classic echo cancellation techniques. In a modem pool environment where several modems transmit via a shared bundle, the canceller for each receiver must take into account all the interfering transmitters.
The addition of a modem to a modem pool that is operating at steady state may cause a significant increase in the noise levels experienced by the currently-operating modems, since the crosstalk generated by the currently-operating modems is already being cancelled, while the crosstalk interference that the new modem causes to the currently-operating modems is not cancelled until its crosstalk function is learned by the currently-operating modems. Until the currently-operating modems learn the crosstalk function of the new modem, the crosstalk generated by the new modem may cause severe degradation in the reception quality of the currently-operating modems, increasing their bit error rate (BER), and possibly causing some currently-operating modems to lose synchronization and deactivate as a result.
The “hitless” addition of a new modem to a modem pool operating at steady state that does not significantly affect the existing noise environment of the modem pool would therefore be advantageous.